Tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72. Early patent literature includes Bogg U.S. Pat. No. 4,063,951, Lewis U.S. Pat No. 4,067,739 and Maternaghan U.S. Pat. No. 4,150,994, U.S. Pat. No. 4,184,877 and U.S. Pat. No. 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of U.S. applications filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described. So Wilgus U.S. Pat. No. 4,434,226 discloses tabular silver bromoiodide grains having a thickness less than 0.3 micron, a diameter of at least 0.6 micron and an average aspect ratio greater than 8:1 and accounting for at least 50 percent of the total projected area of all the emulsion grains. Kofron U.S. Pat. No. 4,439,520 discloses similar grains which are spectrally sensitized. Abbott U.S. Pat. No. 4,425,425 describes radiographic materials containing tabular grains with aspect ratio of at least 8:1 and Abbott U.S. Pat. No. 4,425,426 discloses similar grains with an aspect ratio between 5:1 and 8:1. Solberg U.S. Pat. No. 4,433,048 protects tabular silver bromoiodide grains with inhomogeneously distributed iodide. A survey on high aspect ratio silver halide emulsions appeared in Research Disclosure, Vol 225, Jan 1983, item 22534.
High aspect ratio tabular grains exhibit several pronounced photographic advantages. Thanks to their particular morphology greater amounts of spectral sensitizers can be adsorbed per mole silver halide compared to classical globular grains. As a consequence such spectrally sensitized tabular grains show an improved speed-granularity relationship and a wide separation between their blue speed and minus blue speed. Sharpness of photographic images can be improved using tabular grains thanks to their lower light scattering properties again compared to conventional globular emulsion grains. In color negative materials the conventional sequence of the light sensitive layers can be altered and the yellow filter layer can be omitted. In developed black-and-white images high covering power is obtained even at high hardening levels; alternatively reduced silver halide coverages can be achieved if wanted resulting again in improved sharpness. In double coated radiographic materials the presence of tabular grains reduces the so-called cross-over which is the dominant factor for sharpness in such materials.
The above cited references on tabular grains are mainly concerned with high sensitive silver bromide or silver iodobromide emulsions. However in a lot of photographic applications high sensitivity is of less importance. In these cases the use of chloride rich emulsions is advantageous e.g. thanks to their higher development and fixing rates. Typical examples include graphic arts materials, duplicating materials, radiographic hard-copy materials, diffusion transfer reversal materials and black-and-white or color print materials. So it would be interesting to try to combine the advantages of chloride rich emulsions with the advantages of tabular grain structure.
When using conventional precipitation conditions chloride rich emulsion grains show a cubic morphology with (100) crystal faces. It is known that to alter this crystallographic habit so-called "growth modifiers" or "crystal habit modifiers" are required. Klein and Moisar, in Berichte der Bunsengeselschaft 67 (4), p. 349-355, report inhibition of the growth rate of silver chloride by purine bases such as adenine. Claes et al., J. Photogr. Sci. Vol. 21 (1973), p. 39-50, showed that growth modifiers can be used to precipitate octahedral and rhombic dodecahedral silver chloride crystals and they attributed the crystal habit modification to variations in surface hydration caused by those additives; typical examples of these modifiers included adenine, thiourea, hypoxanthine, benzimidazole and benzothiazole derivatives. The mechanism of the growth modifying action of adenine was studied in detail by Szucs in J. Signal AM Vol. 6 (1978) No 5 p. 381-405.
In more recent patent applications tabular chloride rich emulsion grains are contemplated. So Wey U.S. Pat. No. 4,399,215 discloses tabular silver chloride with an aspect ratio of at least 8:1 and parallel (111) major crystal faces. Precipitation conditions include the use of ammonia. Rather thick tabular grains are obtained. Wey U.S. Pat. No. 4,414,306 discloses tabular silver chlorobromide grains with at least an annular region where the molar ratio of chloride to bromide ranges up to 2:3.
In U.S. Pat. No. 4,400,463 Maskasky describes the preparation of a new crystallographic form of tabular silver halide grains rich in chloride by performing the precipitation in the presence of a special peptizer having a thioether linkage and an aminoazaindene growth modifier. A preferred growth modifier is adenine. Control example Emulsion 2 describes the failure to produce tabular grains using adenine when the special peptizer is absent and only conventional gelatin is present. In this example the reaction vessel contained a rather high amount of chloride (0.5 molar) before the start of the precipitation and pH was adjusted to 3.0.
Maskasky U.S. Pat. No. 4,713,323 discloses the preparation of thin tabular grains (less than 0.35 micron) by a precipitation technique wherein at least a molar concentration of chloride ion is present in the reaction vessel at the start and oxidized gelatin is used containing less than 30 micromoles of methionine per gram. In a preferred embodiment, illustrated by examples, a growth modifier e.g. an aminoazaindene like adenine is used. Control Example 1D describes the failure to produce a tabular AgClBr (1.0% Br) emulsion in the presence of adenine but using conventional gelatin instead of oxidized gelatin. In this case the reaction vessel contained 0.5 molar chloride ions and pH was adjusted to 4.0.
Tufano U.S. Pat. No. 4,804,621 describes a process for preparing chloride rich tabular grains in the presence of aminoazapyridine growth modifiers represented by a general formula of which adenine and derivatives are excluded. A preferred compound is e.g. 4-aminopyrazolo[3,4, d]pyrimidine. Example Control 2 demonstrates the failure to prepare tabular grains using adenine as a growth modifier. In this example pCl was maintained at 0.7 and pH at 4.0.
It is an object of the present invention to provide a new method of preparing tabular silver halide grains rich in chloride.
It is a further object of the present invention to provide photographic materials which contain tabular grains prepared by this new method.
Other objects will become apparent from the description hereafter.